This invention relates to semiconductor devices having an optically-sensitive barrier region which is substantially fully depleted at zero bias and in the absence of incident light, and in particular to such devices which are switchable between a high voltage blocking state and a high current state by means of incident light.
U.S. Pat. No. 4,149,174 discloses several forms of semiconductor devices comprising a semiconductor body having first and second regions of one conductivity type and a barrier region located between the first and second regions to control current flow along a current path between the first and second regions. The barrier region contains a net impurity concentration of the opposite conductivity type and is at least substantially fully depleted of free charge carriers by depletion layers formed with the first and second regions at zero bias, at least in the absence of incident light. These devices are of the unipolar type in which the current flow through the barrier region is by majority carriers (i.e. charge carriers of said one conductivity type) and so can have a high switching speed.
FIG. 6 of U.S. Pat. No. 4,149,174 illustrates a photon detector as one form of such a device. In this case the barrier region is optically-sensitive, and optically transmissive means (in the form of a window in an electrode layer) permit incident light to generate charge carriers in the vicinity of the barrier region to reduce the barrier height of the barrier region. Minority carriers (i.e. charge carriers of said opposite conductivity type) generated in electron-hole pairs by the light are collected in the barrier region thereby modulating the barrier height and the magnitude of the majority carrier current flow through the barrier region. It is suggested that the FIG. 6 device structure may be used as a low photon level detector with high quantum efficiency and gain.
In most of the embodiments shown in U.S. Pat. No. 4,149,174 both the first and second regions adjoin a surface of the body, and the first region is laterally bounded by a single undepleted annular zone of the opposite conductivity type which laterally separates the first region from part of said second region. This annular zone extends sufficiently deeply in the body to intersect the edge of the barrier region so as to terminate the barrier region laterally in the annular zone and thereby to act also as a guard ring for the barrier region. In some embodiments (including the FIG. 6 photon detector) this annular zone is formed in the same doping step used to form the barrier region and extends to the same depth in the body as the barrier region. However in other embodiments the annular zone is provided in a separate doping step so as to form a higher barrier, and in this case it extends generally deeper than the barrier region.
U.S. Pat. No. 4,149,174 does not disclose any specific device designed for blocking high voltages, for example voltages of at least 50 volts. Thus, for example, FIG. 5 of U.S. Pat. No. 4,149,174 illustrates a reverse-biased characteristic for such a barrier region up to 16 volts, after which the reverse leakage current begins to increase significantly as a result of a high electrostatic field in the depleted area of the second region adjacent the barrier region. This field pulls the barrier down and results in unacceptably large reverse currents at high voltages.